1. Field of the Invention
The present invention relates to the electronic sensing of shallow buried objects in the ground surface over large areas, and more particularly to maintenance systems for monitoring changing conditions and deterioration in municipal infrastructures over time, and for locating, distinguishing and designating small obscured bits and pieces.
2. Description of the Problems to be Solved
Construction crews seem to be routinely ripping accidently into buried wires, pipes, and vaults in spite of many official programs and laws for them not to dig without checking first. Sometimes these failures are harmless and little damage is done, but other times serious and catastrophic breaches can occur that take lives, inflict injuries, and costs millions of dollars.
Not every bit of our infrastructures are fully mapped and known, the smaller, older bits have often escaped being inventoried, cataloged and registered. The problems are especially pronounced in third world countries, war-torn areas subject to quick fixes and unauthorized construction, and areas with long histories of historical use.
Pipes, wires, canisters, tubes, and other manufactured items are made of a variety of materials that will electromagnetically or dielectrically contrast with surrounding soils if buried in the earth.
The trouble is many naturally occurring things can appear to a scanner to be manmade objects that have been recently placed and that were not present in previous scans. For example, a heavy overnight dew can produce puddles of conductive water that will contrast with a more dielectric soil. The shapes these can take often mimic objects of interest.
The present inventors discovered that a resonant microwave patch antenna (RMPA) driven by continuous wave signals kept at resonance can be used as a very sensitive sensor. Changes in the dielectric character of the immediate environment will affect the loading on the RMPA and therefore manifest as changes in its complex input impedance at resonance. Movements and stationary anomalies deep in the ground can be sensed and characterized.
Various kinds of conventional, mobile, portable equipment and methods are in use worldwide to scan the top layers and surfaces of the ground to detect and locate valuable, vulnerable, or hazardous objects. Meteorite hunters routinely use tuned coils sensitive to magnetic materials to sweep for melted and burnt bits of iron and steel that fall to the ground from meteors. Other kinds of metal detectors and ground scanners are used by treasure hunters to find hordes of gold, silver, copper and other precious metals and artifacts. Some are now even equipped with video eyeglasses for three dimensional visualizations.
See:www.okmmetaldetectors.com/products/earthimager/exp5000.php
Radar based detectors universally send either continuous wave (CW) or pulsed radio transmissions into the ground so that echoes returned from buried objects can be received and interpreted. As we have described in several of our earlier United States Patents, the so-called “first-interface” between the atmosphere and the top surface of the ground will return a very strong reflection. Such first-interface reflections can completely swamp and obscure the much fainter signals being reflected by small, shallow objects, and/or ones very deeply buried.
Manufactured objects buried in the ground can be constructed as all-plastic, low-metal, and all-metal. The smaller they are overall, the more difficult they will be to detect because they return fainter signals. All-plastic and low-metal objects can avoid or frustrate detection by conventional methods that depend on the presence of iron and an electromagnetic response. Sensors deployed to find these more common objects therefore need to operate in many kinds of modes. The dielectric contrasts of these devices with the surrounding soils can be used to advantage to highlight the object for analysis of signature characteristics.
One-pass and two-pass methods have been conventionally used to find wires, pipes, and other objects-of-interest. One-pass methods must be used when there has been no previous opportunity to make and record a prior sweep or survey.
Two-pass methods can have advantageous benefits, but a preliminary sweep of the ground must be collected to determine one or more baseline conditions. Fortunately, rugged high capacity storage media is now widely available at an economic price. New data from subsequent sweeps can be compared to data from the prior sweeps to highlight any changes, and this advantageous technique is broadly referred to as “change detection”.
Many changes can be quite benign or of no interest whatsoever, e.g., puddles from a recent rain, morning dew, roadside debris, construction, ruts, footprints, and other ordinary events. Very few changes will actually signal something of interest has been detected. Often what is of interest are things that were deliberately placed in the interim, and such indications are too important to be missed or misinterpreted.
An artifact clutter can occur at the surface that will trigger many false positives. Too many false positives will discredit the equipment, and its operators will grow not to trust the reports.
In the one-pass method, virtual nicks and barbs will appear in a three dimensional spiral function that plots real and imaginary RMPA impedances against depth. Homogeneous soils and other materials will return a smooth function resonant detection impedance spiral.
Plastic or metal objects that differ in their dielectric characteristics from the surrounding soils will manifest electronically as inward or outward spikes on the spiral function. Software interprets these spikes as possible objects and their magnitude are telltale of their relative sizes.
In the two-pass method, ground-penetrating-radar (GPR) data is registered to location reference system for later comparison with subsequent passes. The scans from each run are compared, and new objects found in the comparisons are highly probable to be some type of object or preparation worthy of further investigation or avoidance.
Innocent and natural changes can occur in the ground surface between a first and a second pass. For example, changes in moisture can occur that result from rain, dew, and afternoon drying in the sun. Foot and vehicle traffic can leave behind soil compressions and puddles that were not there on the first pass. Such innocent and natural changes have created an intolerable number of false positives in conventional equipment.
There is a need for a device and system that can electronically detect and characterize things around roadways and through the ground, but which has a much lower false alarm rate.